Kafka connect is sending a malformed json - apache-kafka

Question

I'm trying to perform a proof of concept using kafka-connect with a rabbitMQ connector. Basically, I have two simple spring boot applications; a RabbitMQ producer and a Kafka consumer. The consumer can not handle the messages from the connector because it's transforming somehow my JSON message; RabbitMQ sends {"transaction": "PAYMENT", "amount": "$125.0"} and kafka-connect prints X{"transaction": "PAYMENT", "amount": "$125.0"}. Please note the X at the beginning. If I add a field, let's say "foo": "bar" then that letter becomes a t or whatever.
Dockerfile (connector):
FROM confluentinc/cp-kafka-connect-base:5.3.2
RUN confluent-hub install --no-prompt confluentinc/kafka-connect-rabbitmq:latest
Please generate the image as follows: docker build . -t rabbit-connector, so you can reference it in the docker-compose file as rabbit-connector.
docker-compose.yml:
version: '2'
networks:
kafka-connect-network:
driver: bridge
services:
zookeeper:
image: confluentinc/cp-zookeeper:5.3.2
networks:
- kafka-connect-network
ports:
- '31000:31000'
environment:
ZOOKEEPER_CLIENT_PORT: 2181
ZOOKEEPER_TICK_TIME: 2000
KAFKA_JMX_HOSTNAME: "localhost"
KAFKA_JMX_PORT: 31000
kafka:
image: confluentinc/cp-enterprise-kafka:5.3.2
networks:
- kafka-connect-network
ports:
- '9092:9092'
- '31001:31001'
depends_on:
- zookeeper
environment:
KAFKA_BROKER_ID: 1
KAFKA_ZOOKEEPER_CONNECT: zookeeper:2181
KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: PLAINTEXT:PLAINTEXT,PLAINTEXT_HOST:PLAINTEXT
KAFKA_INTER_BROKER_LISTENER_NAME: PLAINTEXT
KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://kafka:29092,PLAINTEXT_HOST://localhost:9092
KAFKA_AUTO_CREATE_TOPICS_ENABLE: "true"
KAFKA_METRIC_REPORTERS: io.confluent.metrics.reporter.ConfluentMetricsReporter
KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1
KAFKA_GROUP_INITIAL_REBALANCE_DELAY_MS: 100
CONFLUENT_METRICS_REPORTER_BOOTSTRAP_SERVERS: kafka:29092
CONFLUENT_METRICS_REPORTER_ZOOKEEPER_CONNECT: zookeeper:2181
CONFLUENT_METRICS_REPORTER_TOPIC_REPLICAS: 1
CONFLUENT_METRICS_ENABLE: 'false'
CONFLUENT_SUPPORT_CUSTOMER_ID: 'anonymous'
KAFKA_JMX_HOSTNAME: "localhost"
KAFKA_JMX_PORT: 31001
schema-registry:
image: confluentinc/cp-schema-registry:5.3.2
depends_on:
- zookeeper
- kafka
networks:
- kafka-connect-network
ports:
- '8081:8081'
- '31002:31002'
environment:
SCHEMA_REGISTRY_HOST_NAME: schema-registry
SCHEMA_REGISTRY_KAFKASTORE_CONNECTION_URL: zookeeper:2181
SCHEMA_REGISTRY_JMX_HOSTNAME: "localhost"
SCHEMA_REGISTRY_JMX_PORT: 31002
rabbitmq:
image: rabbitmq
environment:
RABBITMQ_DEFAULT_USER: guest
RABBITMQ_DEFAULT_PASS: guest
RABBITMQ_DEFAULT_VHOST: "/"
networks:
- kafka-connect-network
ports:
- '15672:15672'
- '5672:5672'
kafka-connect:
image: rabbit-connector
networks:
- kafka-connect-network
ports:
- '8083:8083'
- '31004:31004'
environment:
CONNECT_BOOTSTRAP_SERVERS: "kafka:29092"
CONNECT_REST_PORT: 8083
CONNECT_GROUP_ID: compose-connect-group
CONNECT_CONFIG_STORAGE_TOPIC: docker-connect-configs
CONNECT_OFFSET_STORAGE_TOPIC: docker-connect-offsets
CONNECT_STATUS_STORAGE_TOPIC: docker-connect-status
CONNECT_KEY_CONVERTER: org.apache.kafka.connect.storage.StringConverter
CONNECT_VALUE_CONVERTER: io.confluent.connect.avro.AvroConverter
CONNECT_VALUE_CONVERTER_SCHEMA_REGISTRY_URL: 'http://schema-registry:8081'
CONNECT_INTERNAL_KEY_CONVERTER: "org.apache.kafka.connect.json.JsonConverter"
CONNECT_INTERNAL_VALUE_CONVERTER: "org.apache.kafka.connect.json.JsonConverter"
CONNECT_REST_ADVERTISED_HOST_NAME: "kafka-connect"
CONNECT_LOG4J_ROOT_LOGLEVEL: "ERROR"
CONNECT_LOG4J_LOGGERS: "org.apache.kafka.connect.runtime.rest=WARN,org.reflections=ERROR"
CONNECT_CONFIG_STORAGE_REPLICATION_FACTOR: "1"
CONNECT_OFFSET_STORAGE_REPLICATION_FACTOR: "1"
CONNECT_STATUS_STORAGE_REPLICATION_FACTOR: "1"
CONNECT_PLUGIN_PATH: /usr/share/java,/usr/share/confluent-hub-components
KAFKA_JMX_HOSTNAME: "localhost"
KAFKA_JMX_PORT: 31004
depends_on:
- zookeeper
- kafka
- schema-registry
- rabbitmq
rest-proxy:
image: confluentinc/cp-kafka-rest:5.3.2
depends_on:
- zookeeper
- kafka
- schema-registry
networks:
- kafka-connect-network
ports:
- '8082:8082'
- '31005:31005'
environment:
KAFKA_REST_HOST_NAME: rest-proxy
KAFKA_REST_BOOTSTRAP_SERVERS: 'kafka:29092'
KAFKA_REST_LISTENERS: "http://0.0.0.0:8082"
KAFKA_REST_SCHEMA_REGISTRY_URL: 'http://schema-registry:8081'
KAFKAREST_JMX_HOSTNAME: "localhost"
KAFKAREST_JMX_PORT: 31005
schema.avsc:
{
"type": "record",
"name": "CustomMessage",
"namespace": "com.poc.model",
"fields": [
{
"name": "transaction",
"type": "string"
},
{
"name": "amount",
"type": "string"
}
]
}
So here I am using a StringConverter for my key (which I don't care to be honest) and AvroConverter for the value. Maybe I am missing something or I'm misconfiguring my kafka-connect worker.
My connector configuration is (connector-config.json):
{
"name" : "rabbit_to_kafka_poc",
"config" : {
"connector.class" : "io.confluent.connect.rabbitmq.RabbitMQSourceConnector",
"tasks.max" : "1",
"key.converter":"org.apache.kafka.connect.storage.StringConverter",
"value.converter": "io.confluent.connect.avro.AvroConverter",
"value.converter.schema.registry.url": "http://schema-registry:8081",
"kafka.topic" : "spectrum-message",
"rabbitmq.queue" : "spectrum-queue",
"rabbitmq.username": "guest",
"rabbitmq.password": "guest",
"rabbitmq.host": "rabbitmq",
"rabbitmq.port": "5672",
"rabbitmq.virtual.host": "/"
}
}
To register my connector I do curl -i -X POST -H "Accept:application/json" -H "Content-Type:application/json" http://localhost:8083/connectors/ -d #connector-config.json.
Once I configure everything, I run the following command to print out my messages:
kafka-avro-console-consumer --bootstrap-server localhost:9092 \
--topic spectrum-message \
--from-beginning
And the JSON starts with a letter, so my question is why is this happening? I think something is encoding my message but my rabbitMQ producer is sending a plain JSON message. I can confirm by testing with a RabbitMQ consumer and debugging my application to the point where the message is being sent out.

Answer 1

You need to use the ByteArrayConverter. It's just bytes that the connector pulls from RabbitMQ - it won't try to coerce it to a schema. Even if you serialise it to Avro, the schema is just a single field of bytes:
$ curl -s -XGET localhost:8081/subjects/rabbit-test-avro-00-value/versions/1 | jq '.'
{
"subject": "rabbit-test-avro-00-value",
"version": 1,
"id": 1,
"schema": "\"bytes\""
}
If you want to write it to a topic in Avro (which is a good idea) with a schema then use something like Kafka Streams or ksqlDB to do this, applying a stream processor to the source topic which Kafka Connect writes to with the ByteArrayConverter.
For example in ksqlDB you would do:
-- Inspect the topic - ksqlDB recognises the format as JSON
ksql> PRINT 'rabbit-test-00' FROM BEGINNING;
Format:JSON
{"ROWTIME":1578477403591,"ROWKEY":"null","transaction":"PAYMENT","amount":"$125.0"}
{"ROWTIME":1578477598555,"ROWKEY":"null","transaction":"PAYMENT","amount":"$125.0"}
-- Declare the schema
CREATE STREAM rabbit (transaction VARCHAR,
amount VARCHAR)
WITH (KAFKA_TOPIC='rabbit-test-00',
VALUE_FORMAT='JSON');
-- Reserialise to Avro
CREATE STREAM TRANSACTIONS WITH (VALUE_FORMAT='AVRO',
KAFKA_TOPIC='reserialised_data') AS
SELECT *
FROM rabbit
EMIT CHANGES;
For more details, see this blog that I've written up.

Answer 2

You don't have JSON messages, you have Avro messages coming out of Kafka based on the usages of the AvroConverter.
That letter is not actually a letter, but your terminal showing the UTF8 representation of the first 5 bytes of the binary data. That commonly happens when using regular console consumer, rather than the avro-console-consumer which itself will parse the bytes out of the topic correctly for Avro data
If you want JSON throughout, use the JSONConverter instead